Conventionally, sodium hypochlorite solution having sterilizing properties has been used as a bleaching agent or a mold remover. Also, the use of chlorine dioxide for sterilization of city water has been studied. Such sterilizing water in which the concentration of chlorine is increased as compared with city water etc. to enhance the sterilization power has been used at job sites requiring sterilization, for example, for HACCP-compatible food processing, stock raising such as breeding of SPF pigs, and medical care.
It is known that the bactericidal action of the above-described chlorine-based compound changes greatly according to the state of the chlorine-based compound in aqueous solution, and depends heavily on the acidity (pH). Especially in the case of sodium hypochlorite, bactericidal action differs greatly depending on the pH range, between a strongly acidic condition, a weakly acidic through neutral condition, and an alkaline condition.
In the strongly acidic condition (where pH is lower than 3.8), chlorine gas is liberated from sodium hypochlorite solution. This chlorine gas is toxic, and hence the application of the manufactured sterilizing water is undesirably limited to a substantial extent, though the sterilizing water has a bactericidal action. Also, in the alkaline condition (where pH exceeds 7.5), the ratio of ionization of chlorine in the solution into the form of hypochlorous acid ion (OCl—) increases. The hypochlorous acid ion has weak sterilization power, being about 1/80 of the sterilization power of hypochlorous acid (HOCl) with the same chlorine concentration. Therefore, to raise the sterilization power, the chlorine concentration must inevitably be increased. However, even if sterilization power is raised by increasing the concentration, though sterilization power rises, the concentration has to be increased still further, since sodium hypochlorite solution itself is alkaline.
On the other hand, in the weakly acidic through neutral condition (where pH is in the range of 4.8 to 7.5), a large amount of chlorine takes the form of hypochlorous acid (HOCl), so that the sterilization power can desirably be raised without the production of chlorine gas. For example, Japanese Patent Provisional Publication No. 10-182325 (No. 182325/1998) has discloses a “Device for Reinforcing Sterilization Power of Sodium Hypochlorite”. This Document discloses that either acid or chlorine-based solution is diluted and fed to a water flow by separate feeders. In this disclosure, acid solution or chlorine-based solution is poured into a flow path by a pump. A configuration is mainly disclosed, in which hypochlorous acid is fed to the water flow from a chlorine-based solution tank by a pump, and subsequently acid is fed by a pump. Further, in mixing, mixing means consisting of an agitator or the like is used.
However, the apparatus and method for manufacturing sterilizing water disclosed until now have the following problems:
(1) In the publicly known configuration, an acid is used to make the liquid into a weakly acidic condition, wherein the finally manufactured sterilizing water, chlorine gas is liable to be produced due to nonuniform concentration of acid or hypochlorous acid.
(2) In order to control the acidic water to which sodium hypochlorite solution is fed so as to provide a proper acidity, a precise control system is needed, and also, a plurality of feeders are needed. Thus, the apparatus becomes large in scale, and the installation location is limited, resulting in a high cost.
(3) Also, a pump is used to feed acid or sodium hypochlorite, meaning that the pump must be controlled with high accuracy.
Furthermore, while an agitator etc. could also be used in a mixer, such a configuration has the following problems:
(4) A mixer which comprises an agitator or the like desirably has a fixed shape in order to mix the liquid efficiently, but the material thereof is limited because the mixer itself is used to mix solutions in a wide range of acidity conditions, from acidic condition to alkaline condition. In particular, when a resin is used to make a mixer, no suitable mixer is known in terms of acid resistance and alkali resistance.
(5) Where the flow path must be straight in order to arrange the mixer in the flow path, the design of the whole of the apparatus is severely restricted, and making the equipment smaller is hindered.
Furthermore, a Venturi type feeding method, which utilizes negative pressure, could also be adopted for feeding acid or hypochlorous acid according to the quantity of production per unit time of the sterilizing water. However, this method has the following problems:
(6) Where a pump for producing a water flow is arranged on the upstream side of a mixer, acidic water is produced by a Venturi type feeder and further sodium hypochlorite solution is fed by the Venturi type feeder, and a faucet etc. are provided on the downstream side thereof as an outlet for the sterilizing water. In such a case, since the degree of opening of the faucet can be adjusted arbitrarily, acid solution and sodium hypochlorite solution must be fed in proportion to the current flow rate according to the degree of opening of the faucet.
(7) With this method, it is necessary to precisely control a very low flow rate of the feed chemical. If the flow rate is controlled by using a needle type flow regulator, the shape of an opening for restricting the flow rate becomes a doughnut shape or the like having a very small cross section, so that the flow rate is varied by the deformation of flow regulator due to thermal expansion of material of the flow regulator caused by a change in environmental temperature. This phenomenon is particularly prominent when the flow regulator is made of a resin to ensure chemical resistance. Further, if small quantities of dust etc. are contained in the flowing liquid, the flow path is soon clogged, which hinders stable feeding operation.
(8) The negative pressure for this suction feeding operation is approximately 100 to 1000 mm in water-gauge pressure (=about 980 to 9800 Pa=about 0.00968 to 0.0968 atm). This negative pressure is at the same level as the pressure produced by a difference in height of water of about 10 cm. Therefore, the flow rate of the suction fed liquid is affected by a minute change in pressure of water flow that achieves suction.
(9) When the whole of the apparatus is in actual use, it is favorable to provide a safety device that monitors the quantity of feeding of acid or hypochlorous acid as appropriate so that the supply of sterilizing water is shut off when the flow rate exceeds a certain value. In this case, the manufacture path of sterilizing water can be shut off by using an electromagnetic valve etc. as the safety device. However, if the flow rate in the supply path of acid or hypochlorous acid is used as the judgment criterion, a flow sensor is needed to detect the flow rate in the supply path of acid or hypochlorous acid. An inexpensive flow sensor that has high chemical resistance and is capable of detecting a very low flow rate sensitively is not yet known.
(10) Furthermore, if the path to be detected contains air bubbles, the flow sensor generally produces an error. When the apparatus for manufacturing sterilizing water is in actual use, it is favorable to maintain a state in which air bubbles do not intrude into the path during the operation and shutdown of the apparatus.
(11) Also, when acidic solution or chlorine-based solution is fed at a low flow rate, even if it is attempted to directly regulate the quantity of feeding thereof, dust etc. contained in the solution exerts an influence on the regulating portion, so that it is not easy to perform stable operation for a long period of time.